[SPARK-8174] [SPARK-8175] [SQL] function unix_timestamp, from_unixtime

unix_timestamp(): long
Gets current Unix timestamp in seconds.

unix_timestamp(string|date): long
Converts time string in format yyyy-MM-dd HH:mm:ss to Unix timestamp (in seconds), using the default timezone and the default locale, return null if fail: unix_timestamp('2009-03-20 11:30:01') = 1237573801

unix_timestamp(string date, string pattern): long
Convert time string with given pattern (see [http://docs.oracle.com/javase/tutorial/i18n/format/simpleDateFormat.html]) to Unix time stamp (in seconds), return null if fail: unix_timestamp('2009-03-20', 'yyyy-MM-dd') = 1237532400.

from_unixtime(bigint unixtime[, string format]): string
Converts the number of seconds from unix epoch (1970-01-01 00:00:00 UTC) to a string representing the timestamp of that moment in the current system time zone in the format of "1970-01-01 00:00:00".

Jira:
https://issues.apache.org/jira/browse/SPARK-8174
https://issues.apache.org/jira/browse/SPARK-8175

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #7644 from adrian-wang/udfunixtime and squashes the following commits:

2fe20c4 [Daoyuan Wang] util.Date
ea2ec16 [Daoyuan Wang] use util.Date for better performance
a2cf929 [Daoyuan Wang] doc return null instead of 0
f6f070a [Daoyuan Wang] address comments from davies
6a4cbb3 [Daoyuan Wang] temp
56ded53 [Daoyuan Wang] rebase and address comments
14a8b37 [Daoyuan Wang] function unix_timestamp, from_unixtime